The present invention pertains generally to measuring and testing devices and more particularly to photoionization detectors.
For the past few years, there has been a great deal of effort devoted to developing an improved device for detecting ultraviolet light such as disclosed by A. Policarpo, Nucl. Instr. and Meth. 153, 389 (1978); G. Charpak et al., IEEE Trans. Nucl. Sci. NS-27, 212 (1980); and D. F. Anderson, IEEE Trans. Nucl. Sci. NS-27, 181 (1980). The primary interest in developing an improved ultraviolet light detector is for the purpose of replacing photomultiplier tubes used in conjunction with xenon filled gas scintillation proportional counters for detecting x-ray radiation. Additionally, there has also been considerable interest in developing an instrument to detect Cerenkov radiation as more fully disclosed in G. Melchart et al., IEEE Trans. Nucl. Sci. NS-27, 124 (1980).
Since the emission spectrum of xenon in xenon filled gas scintillation proportional counters is in the ultraviolet, ranging from approximately 1500-1900 .ANG. and peaking at about 1670 .ANG., photomultiplier tubes with Spectrosil windows have typically been used in conjunction with xenon filled gas scintillation proportional counters for detecting uv photons. In other words, x-rays impinging upon the gas scintillation proportional counter produce ultraviolet photons which are converted by photomultiplier tubes into an electrical signal representative of the energy of the x-ray radiation. These devices have been particularly useful in detecting, and in some cases imaging, low energy x-ray radiation ranging from approximately 0.5 keV to 30 keV.
Although other techniques have been suggested for replacing photomultiplier tubes to detect the ultraviolet light produced by the xenon gas in gas scintillation proportional counters, the most promising device has been the photoionization detector. The photoionization detector is a proportional counter which is filled with an appropriate gas that converts incident ultraviolet photons produced by the gas scintillation proportional counter to electrons through the photoelectric effect. The coupling of a photoionization detector to a gas scintillation proportional counter was first disclosed by Policarpo in Nucl. Instr. and Meth. 153, 389 (1978), wherein he suggests that if a sufficient fraction of the ultraviolet photons produced by the gas scintillation proportional counter can be detected in the photoionization detector, a substantial improvement in energy resolution is possible.
The advantages of the photoionization detector over photomultiplier tubes for detecting light from xenon gas scintillation proportional counters are numerous. Photoionization detectors can be constructed with any desired dimensions and with a variety of uv windows. Additionally, photoionization detectors are more rugged and compact than photomultiplier tubes and can be constructed at a lower cost than photomultiplier tubes having equivalent window areas. Moreover, photoionization detectors are "solar blind," i.e., insensitive to visible radiation, eliminating the problem of light leaks which are prevalent in photomultiplier tube detectors. Furthermore, photoionization detectors find utility in high or varying magnetic fields where gas scintillation proportional counters have been shown to be useful, such as disclosed in M. Fatima et al., IEEE Trans. Nucl. NS-27, 208 (1980), but where photomultiplier tubes have not produced reliable outputs. Additionally, photomultiplier tubes have been found to have a nonuniform quantum efficiency across the window of the photomultiplier tube which causes incorrect data measurements in many cases.
The photoionization detector, on the other hand, has a highly uniform quantum efficiency across the window face, resulting in highly uniform and accurate measurements of data. Moreover, with the use of an imaging photoionization detector, such as disclosed by W. Ku et al., IEEE Transaction NS-28, 830 (1981) and similar to the imaging proportional counters disclosed by P. B. Reid et al., IEEE Trans. Nucl. Sci. NS-26, 46 (1979) and G. Charpak et al., Nucl. Instr. and Meth. 148, 471 (1978), event locations can be determined with high accuracy, for imaging Cerenkov radiation and producing imaging gas scintillation proportional counters.
Charpak, Policarpo, and Sauli have demonstrated a gas scintillation proportional counter coupled to a photoionization detector as disclosed in G. Charpak et al., IEEE Trans. Nucl. Sci. NS-27, 212 (1980). As disclosed therein, a krypton gas scintillation proportional counter was coupled to a photoionization detector filled with 83% argon, 3% TEA (triethylamine), and 14% methane. Although the energy resolution obtained was good (10.8% FWHM at 5.9 keV), a krypton gas scintillation proportional counter has limited applications due to its poor x-ray stopping power and high radioactive background from .sup.85 Kr. Additionally, the photoionization potential of TEA is too high for the detection of xenon light and use of krypton and TEA requires windows such as LiF.sub.2 and MgF.sub.2 which are difficult to work with and expensive.